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RE: Interesting cable problem, need ideas

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In Reply to: RE: Interesting cable problem, need ideas posted by morrowaudio on May 08, 2008 at 04:19:21

yup, that is pretty similar to the infamous "Cobra Cables" from the '70s that would regularly blow up certain solid state amps. I've replaced output transistors in a lot of PhaseLinear 400's because of that kind of cable.

What was made is a very low-inductance cable -- but lowering inductance in ways like that increases capacitance. Basically, you'll never get 1/sqrt(L'C') to be higher than 3E8[m/s] (the speed of light), blame Dr. Einstein for that.

L' and C', by the way, are inductance per meter and capacitance per meter (not just the lumped inductor and capacitor values, though those won't be far off at audio freqencies). The only way to approach the speed of light limit is to use insulators of the lowest dielectric constant (though you won't make any large changes in that velocity even with that). The balance between the inductance and capacitance is the "characteristic impedance" sqrt(L'/C'). The net result is that if you try to make a cable with impedance down near 8 ohms (which people for some reason want to do), it will have very large capacitance (because of the speed of light thing again). Which would be fine if a speaker really had an 8 ohm constant impedance; but most speakers actually go inductive at high frequencies so the amp ends up seeing the capacitance only at high freq which can make them unstable.

You might be able to get it to behave by adding, across the speaker terminals, a 10 ohm resistor in series with a capacitor of about 0.1uF. That would cut the inductance at high frequencies, though the exact result might depend on the speaker impedance.

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